1. Field of The Invention: This invention relates to methods and apparatus for preventing a chimney effect through a ladle additive chute when materials are introduced to molten metal.
2. Prior Art: In the making of steel, additives such as alloying materials and the like are best introduced while molten steel is tapped from a furnace into a ladle. For example, in basic oxygen process furnaces, additives such as sulfur, coke, ferromanganese, and others are introduced from above the furnace and ladle through a chute that opens at its lower end above the ladle adjacent the molten stream of metal that flows from the furnace into the ladle. Not only do the ambient gases heated by the steel rise through the chute at that time, but also the additives themselves may create smoke, fumes or eruptive flames, as with sulfur, that rise through the chute. At times, flames may extend 10 to 12 feet above the chute. Where ladle additions are made manually, this creates a substantial hazard to a workman; when made automatically, certain parts of the equipment adjacent the chute inlet, such as load cells for weighing the additives, may be damaged by the heat. In effect, this condition has made it difficult or impossible to make ladle additions at the proper time during tap without taking some measures to retard the back draft.
Attempts to vary the addition technique, as by placing the additive in the empty ladle and then tapping the furnace have resulted in a poor mix and segregation of alloying materials in the heat. Mechanical swinging gates or dampers have been used in chutes to close off back drafts, but have the disadvantage of being subject to failure due to their mechanical nature and the unfavorable environment, and may block the chute at a critical time. Separate evacuation systems to divert the back drafts have also been attached to chutes, but have been expensive.
In a different environment, charging systems for furnaces have been shown that create a static gas seal at a charging opening using scrubbed gas from the furnace to prevent dirty gas from within the furnace from escaping. See U.S. Pat. No. 3,198,623. Such gas from the furnace is combustible and the arrangement is neither structurally nor functionally satisfactory for a ladle additive chute. In particular, combustible gas would ignite and a static seal is not practical or sufficiently effective for present purposes.